1. Field of the Invention
The present invention relates generally to sensors, and in particular to a nondestructive method of utilizing piezoelectric sensors to determine in situ the health of solid rocket propellants.
2. Related Art
There is a need for an apparatus for measuring the health of solid rocket propellant using an embedded sensor for self-sensing ordnance. The need is particularly acute with regard to solid rocket motors, since it is known that aging of propellant can lead to significant degradation in weapon performance and, possibly, catastrophic failure. The need correlates with the military requirements that mandate future tactical missiles be kept for extended periods of time on board ship, without periodic, land-based inspections. A particularly facile method of inspection would be one where the solid rocket missiles have a solid rocket motor with a propellant that has a method of self-sensing the health of the propellant.
Classical approaches used to predict and detect material degradation have been to develop aging models for predicting the state of a material, given an assumed or measured environmental history, and the use of non-destructive testing methods such as ultrasound and X-rays. Both approaches, as currently practiced, are inadequate to meet the needs of a real-time, self-sensing health monitoring system. Thus, in recent years efforts have been devoted to investigate an entirely new approach to meet the goal of self-sensing ordnance—the use of embedded sensors.
The use of embedded sensors is potentially a better analytical technique for several reasons. A sensor embedded in the propellant inside a weapon, versus an external technique that is obstructed by a thick metallic housing, is in direct contact with the energetic material, and thus in a better position to detect subtle changes in properties. An embedded sensor is always present in the weapon, and thus the weapon's health can always be queried, thus meeting the goal of making the ordnance self-sensing.
Several types of embedded sensors are being investigated in the solid rocket motor community. Bond line sensors are small pressure sensors used to measure the stress between the propellant and case. The sensors are used to detect the perturbation in the stress field due to the presence of damage. Difficulties with this method are as follows. The bond line sensor requires sophisticated finite element modeling and analysis to characterize the damage from the measured signals. The analysis is further complicated by the unknown change in material properties due to aging, leading to problems in data interpretation. Bond line sensors are difficult to install, as they need to be cast into the rocket motor, and they have relatively high cost, on the order of $250 per sensor. A second type of sensor is an optical fiber strain sensor. Optical fiber strain sensors are used in a similar manner to bond line sensors, in that they detect changes in the strain field due to the presence of damage. While optical fiber sensors can be placed in the bore, and thus can be installed after the motor is cast, the difficulty in interpreting the signals is a significant challenge.
A weakness with both these sensors is that they do not provide an unambiguous indication of the system's health, and significant analysis is needed to interpret the results.
What is needed is a reliable measure of the health of the solid rocket propellant through the use of an embedded sensor, where the method provides an unambiguous measure of material state, where the method employs an embedded sensor that is relatively inexpensive.